0040 - Select type support

• Planned Version: 202Y

Introduction

This proposal seeks to extend the select intrinsic in Clang’s HLSL implementation to support additional result types beyond scalar, vector, and matrix types (e.g. structs).

Motivation

The select intrinsic was introduced in HLSL 2021 to replace the non-short-circuiting vector ternary operators that existed in earlier versions of HLSL. In DXC, this pre-HLSL-2021 ternary operator only supported scalar, vector, matrix, and SamplerState result types. DXC’s select implementation mirrors these restrictions.

While Clang aims to match HLSL intrinsic behaviors to DXC, it wouldn’t make much sense for Clang’s select implementation to have the same restrictions as DXC’s. HLSL in Clang aims to align more closely with C++ conventions, and given that the C++ ternary operator works with more general types, it would be reasonable for Clang’s select to be similarly general.

Proposed solution

Allow Clang’s select intrinsic to support more types outside of the ones supported by DXC, such as structs. This support would apply through the scalar degenerate case:

template <typename T>
_HLSL_BUILTIN_ALIAS(__builtin_hlsl_select)
T select(bool, T, T);

Note: Since HLSL does not support array return types from functions, select would not support arrays.

This proposal does not suggest making similar changes to DXC, as DXC’s ternary operator intentionally limits its supported types. Clang’s divergence here is justified by its alignment with C++ semantics.